The present invention relates generally to lasers based on chiral structures, and more particularly to electronically or optically pumped lasers utilizing cholesteric liquid crystal elements.
Semiconductor lasers have found many industrial and commercial applications in recent years. For example lasers are used in telecommunications, in optically readable media pickups that are used in CD players, CD ROM drives and DVD players, in medical imaging, and in video displays. However, previously known semiconductor lasers have a number of disadvantages. For example, traditional semiconductor lasers, such as ones used in CD players, emit light from the edge of a chip, so it is necessary to cleave a wafer into chips and package the chip before knowing if the laser functions properly. Other types of light sources, such as LEDs do not provide the performance needed for certain applications.
Vertical Cavity Surface Emitted Lasers (hereinafter xe2x80x9cVCSELsxe2x80x9d) have been developed to address the need for a more advanced, higher quality laser that can function well in a variety of applications. VCSELs combine the performance advantages of LEDs and edge-emitting lasers at costs comparable to LED solutions. VCSELs emit light vertically from the wafer surface, like LEDs, which means their fabrication and testing is fully compatible with standard I.C. procedures and equipment, and also means that arrays of VCSELs are feasible. Additionally, VCSELs are much faster, more efficient, and produce a smaller divergence beam than LEDs.
The VCSEL structure leads to a host of performance advantages over conventional semiconductor lasers.
1) small size
2) low power consumption
3) 2-dimensional array capabilities
In contrast to conventional edge-emitting semiconductor lasers, the surface-emitting VCSEL has a radially symmetric Gaussian near-field, greatly simplifying coupling to optical elements or fibers. In addition, VCSEL technology allows the fabrication of two-dimensional laser arrays.
However, VCSELS suffer from a number of disadvantages. The manufacture of VCSELs requires sophisticated and expensive mircofabrication. Since single-pass gain in thin layer semiconductor lasers is low, VCSELs incorporate highly reflective dielectric stacks which are integrated into the laser as Bragg reflectors (simulating a Distributed Feedback Laser). These consist of alternating layers of dielectric material, which are grown using methods of molecular beam epitaxy (MBE). This ensures a close match of the atomic lattice structures of adjacent layers. Alternating atomically ordered layers of materials with different electronic characteristics are thereby produced. The interfaces between the layers must be digitally graded and doped to reduce the electrical resistance.
Much work has been done to improve the performance of VCSELs by increasing the number of layers and/or the dielectric difference between alternating layers. However, this approach makes the fabrication more expensive and difficult. There is also a limit to the number of layers determined by the absorption in these layers. While VCSELs can be manufactured in two-dimensional arrays, there has been great difficulty in achieving uniform structure over large areas and in producing large area arrays. The materials typically used for VCSELs do not have the desired low absorption and high index contrast over a broad frequency range. In particular, it is difficult to achieve high reflectivity in the communication band around 1.5 microns.
In addition, VCSELs cannot be tuned in frequency since their periods cannot be changed. The density of photon modes is not changed appreciably by use of a low index contrast multilayer Bragg reflector and the gain cannot be improved in a VCSEL system as compared to that in an ordinary laser cavity. Also, an external device must be used to control the polarization of the light.
It would thus be desirable to provide a laser apparatus and method that has advantageous properties similar but superior to VCSELs and that has none of the VCSELs"" disadvantages.
This invention relates to use of chiral structures combined with an excitable light-emitting material to produce lasing. A chiral laser apparatus comprises a layered structure configured to produce a photonic stop band, the layered structure including a upper chiral material layer, a middle excitable light-emitting layer, and a lower chiral material layer, and an excitation source that, when applied to the layered structure, causes the middle light-emitting layer to emit electromagnetic radiation, such that polarized lasing at a lasing wavelength, within or at an edge of the photonic stop band, is produced in a direction perpendicular to the layered structure. The middle light-emitting layer may be configured to produce a defect such that lasing advantageously occurs at a wavelength corresponding to a localized photonic state within the photonic stop band that preferably corresponds to a location of a maximum energy density within the layered structure. The excitation source may be an electrical power source connected to the layered structure via two or more electrodes. In another embodiment of the invention, the layered structure is replaced with a homogeneous cholesteric material doped with a light-emitting material. Excitation sufficient to cause lasing is provided by the electrical power source via a pair of electrodes connected to the cholesteric material. In yet another embodiment of the invention, the excitation source may be an electromagnetic wave source that applies an electromagnetic wave to the middle light-emitting layer to excite the middle layer sufficiently to cause lasing.
The inventive apparatus and method advantageously overcome the drawbacks of previously known edge-emitting lasers and VCSELs due to unique properties of chiral cholesteric) materials utilized in the various embodiments of the present invention. Specifically, the disadvantages of the prior art are overcome as follows:
1) In contrast to multi-layered structures, such as VCSELs, that are difficult to produce, the CLC films/layers utilized in accordance with the present invention are self-organized structures that are readily fabricated;
2) The period of a CLC film/layer could be readily changed by applying an electric or magnetic field or changing temperature or pressure so that the laser output could be tuned in frequency within the gain band of the light-emitting middle layer;
3) The band structure of a CLC film/layer leads to an increase in the local density of photon modes over wavelength range. This in turn results in an improvement in gain and in a reduction of the lasing threshold; and
4) The polarization of the laser output is determined by the CLC structure. Thus the laser beam is right or left circularly or linearly polarized without requiring any external device.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.